In situ ion microprobe ²⁰⁷Pb/²⁰⁶Pb dating of monazite from Precambrian metamorphic suites, Tobacco Root Mountains, Montana

The Precambrian rocks of the Tobacco Root Mountains have been separated into three suites: the Indian Creek Metamorphic Suite, the Pony-Middle Mountain Metamorphic Suite, and the Spuhler Peak Metamorphic Suite. The Indian Creek and Pony-Middle Mountain Metamorphic Suites are quartzofeldspathic gneiss suites that contain variable amounts of metasupracrustal rocks. The Spuhler Peak Metamorphic Suite contains primarily mafic rocks and is possibly ocean crust. Metamorphosed mafic dikes and sills that intruded the Indian Creek and Pony-Middle Mountain Meta morphic Suites, but not the Spuhler Peak Metamorphic Suite, indicate juxtaposition of the Spuhler Peak Metamorphic Suite with the other two suites after intrusion of the dikes at ca. 2060 Ma. All rocks have been deformed and metamorphosed together, initially at pressures greater than 1.0 GPa and temperatures greater than 750 °C, followed by differential reequilibration at ∼0.6 GPa on a clockwise pressure-temperature path. Two-hundred-seventy-two ²⁰⁷Pb/²⁰⁶Pb spot ages of monazite grains from seventeen Spuhler Peak Metamorphic Suite, five Pony-Middle Mountain Metamorphic Suite, and eight Indian Creek Metamorphic Suite rocks have been obtained from the University of California at Los Angeles ion microprobe. Based on the distribution of the ages, the samples can be divided into three groups. (1) All seventeen Spuhler Peak Metamorphic Suite, one Pony-Middle Mountain Metamorphic Suite, and two Indian Creek Metamorphic Suite samples have relatively homogeneous spot age populations that vary from ca. 1720 to ca. 1780 Ma. (2) A group of seven Indian Creek and Pony-Middle Mountain Metamorphic Suite samples has spot ages from monazite grains that form an array near 2450 Ma. (3) A group of four Indian Creek and Pony-Middle Mountain Metamorphic Suite samples are bimodal in that they contain spot ages from both the 1720-1780 Ma group and the 2450 Ma array. There are younger and a few older spot ages in these samples that likely represent mixed age domains, the former between the 1720-1780 Ma and the 2450 Ma age domains and the latter between older detrital grain cores and the 2450 Ma array. Monazite grains in the matrix have similar ages to those that occur as inclusions in garnet and kyanite. Thus, the monazite in these rocks, as well as the peak metamorphic minerals, either grew or re-equilibrated during the higher-pressure (>1.0 GPa) metamorphism. The near absence of ²⁰⁷Pb/²⁰⁶Pb ages older than 1780 Ma in the Spuhler Peak Metamorphic Suite and the common occurrence of older ages in monazite from the Indian Creek and Pony-Middle Mountain Metamorphic Suites are consistent with assembly of the Tobacco Root terrane during a prolonged (60 m.y. long) collision event, the Big Sky orogeny, beginning at ca. 1780 Ma and culminating at ca. 1720 Ma. The Early Proterozoic Big Sky orogeny significantly overprinted the effects of an earlier ca. 2450 Ma orogeny in both the Pony-Middle Mountain and Indian Creek Metamorphic Suites. This older event modified pre-existing Archean rocks. However, of the 272 spot ages on monazite grains reported here, only six are significantly older than 2450 Ma and only one of these is older than 2600 Ma - a 2988 Ma spot age from a monazite inclusion in a garnet from a Pony-Middle Mountain Metamorphic Suite sample. There is no evidence of widespread Archean events recorded in the monazite grains of the Tobacco Root Mountains.